All ARM instructions are 32 bits long. Instructions are stored word-aligned, so the least significant two bits of instruction addresses are always zero in ARM state.
Thumb instructions are either 16 or 32 bits long. Instructions are stored half-word aligned. Some instructions use the least significant bit of the address to determine whether the code being branched to is Thumb code or ARM code.
Before the introduction of 32-bit Thumb instructions, the Thumb instruction set was limited to a restricted subset of the functionality of the ARM instruction set. Almost all Thumb instructions were 16-bit. Together, the 32-bit and 16-bit Thumb instructions provide functionality that is almost identical to that of the ARM instruction set.
The following table describes some of the functional groupings of the available instructions:
